PHYSICAL CHEMISTRY

N. I. BLINOVA, G. A. ROMANOV, V. M. SOLNTSEV, Yu. M. TOLMACHEV

ON THE MAGNETIC PROPERTIES OF $\mathrm{U}_2\mathrm{O}_5$

(Presented by Academician A. A. Grinberg, 21 VII 1962)

As we have shown ($^1$), the product of the interaction of uranium uranous-uranic oxide with acids in the absence of oxidizing agents is $\mathrm{U}_2\mathrm{O}_5$. It was suggested in this connection that $\mathrm{U}_2\mathrm{O}_5$ is a compound of pentavalent uranium. However, the possibility of another structure of this oxide was not excluded, namely that it is a compound of the type $\mathrm{UO}_2 \cdot \mathrm{UO}_3$, in which one uranium atom is in the 4-valent state and the other in the 6-valent state. We attempted to resolve this question by determining the magnetic moment of the uranium atoms in this compound.

The measurement of the magnetic susceptibility of $\mathrm{U}_2\mathrm{O}_5$ was carried out by the Gouy method ($^2$). The apparatus was calibrated by measuring the specific magnetic susceptibility of water and benzene. The magnetic susceptibility was measured for two preparations of $\mathrm{U}_2\mathrm{O}_5$ obtained by treating uranium uranous-uranic oxide with dilute sulfuric acid. The method of preparing $\mathrm{U}_2\mathrm{O}_5$ will be described in greater detail in a separate paper. In addition, for comparison, the magnetic susceptibilities of uranium dioxide and also of uranium uranous-uranic oxide, from which the $\mathrm{U}_2\mathrm{O}_5$ samples were obtained, were measured. For the measurements, samples of substances weighing about 1–1.5 g were used. The values we obtained for the molar magnetic susceptibilities $\chi_M$ for all the indicated substances are given below in Table 1. In the temperature interval studied (between 6.5 and 60°), the magnetic susceptibilities of these compounds follow the Curie–Weiss law quite accurately (see Fig. 1).

Table 1

Table 1 gives the values of the Weiss constants ($\Delta^\circ$), as well as the magnetic moments ($\mu_{\mathrm{exp}}$) calculated from the susceptibility data, expressed in Bohr magnetons. In calculating the paramagnetic component, a correction was introduced for the diamagnetic susceptibility inherent in each compound, on the basis of the data given in Selwood’s monograph. For $\mathrm{U}_3\mathrm{O}_8$, moreover, the polarization paramagnetism of the $\mathrm{UO}_3$ group ($^3$) was taken into account. The composition of each given sample of $\mathrm{U}_2\mathrm{O}_5$, $\mathrm{UO}_2$, and $\mathrm{U}_3\mathrm{O}_8$ was checked before and after the magnetic measurements. According to chemical analysis, the composition of these samples remained unchanged from the time of preparation to the end of the measurements and corresponded to their formulas.

Discussion of the results. The average value of the magnetic moment of $\mathrm{U}_2\mathrm{O}_5$, equal to 1.74, was obtained from the experimental data under the assumption that $\mathrm{U}_2\mathrm{O}_5$ is a compound of pentavalent uranium. It practically exactly coincides with the theoretical value of the magnetic moment, 1.73, due only to the spin of one unpaired electron. Evidently, in the crystal field of $\mathrm{U}_2\mathrm{O}_5$ there is complete suppression of the orbital component. If it is assumed that $\mathrm{U}_2\mathrm{O}_5$ is a compound of the $\mathrm{UO}_2\cdot\mathrm{UO}_3$ type, the experimental data lead to a value of $\mu_s$ equal to 2.30. This value differs substantially from the theoretical value 2.83 for two unpaired electrons. Thus, our data convincingly show that $\mathrm{U}_2\mathrm{O}_5$ is a compound of pentavalent uranium.

The magnetic moment of $\mathrm{UO}_2$, calculated from our measurement data, as is seen from Table 1, agrees well with the values available in the literature (⁴) and exceeds the value 2.83 by only 6%.

Fig. 1. Curves of the dependence of $\chi_M$ on temperature: I, II—for the $\mathrm{U}_2\mathrm{O}_5$ sample (samples 1 and 2), III—for $\mathrm{U}_3\mathrm{O}_8$

As regards $\mathrm{U}_3\mathrm{O}_8$, the following must be noted. Haraldsen and Bakken (⁵), on the basis of the agreement of the experimentally obtained magnetic moment $\mu = 1.39$ with the theoretical $\mu_s$, equal, in their opinion, to 1.42 for one unpaired electron, came to the conclusion that $\mathrm{U}_3\mathrm{O}_8$ has the structure $\mathrm{UO}_3\cdot\mathrm{U}_2\mathrm{O}_5$. As is known, $\mu_s$ is proportional not to the number of unpaired electrons $(n)$, but to the quantity $\sqrt{n(n+2)}$. This circumstance often leads to incorrect conclusions if it is erroneously assumed that $\mu_s$ for one unpaired electron is half as large as for two. Evidently, it was precisely this error that Haraldsen and Bakken made. Consequently, they should have compared the two quantities 1.39 and 1.73, and, of course, as a result of this they would not have arrived at the conclusion indicated above. Meanwhile, the value 1.42 and the conclusions of Haraldsen and Bakken are cited, for example, by Katz and Rabinowitch (⁶).

The value of $\mu_s$ obtained by us for $\mathrm{U}_3\mathrm{O}_8$, equal to 1.59, differs from the value 1.73 by only 9%. Therefore, in the present case the conclusion concerning the presence of pentavalent uranium atoms in the structure of $\mathrm{U}_3\mathrm{O}_8$ appears better substantiated. The comparatively large value of the Weiss constants for $\mathrm{U}_2\mathrm{O}_5$, $\mathrm{UO}_2$, and $\mathrm{U}_3\mathrm{O}_8$ possibly indicates a strong exchange interaction between uranium ions in these oxides.

In conclusion, the authors express their gratitude to Corresponding Member of the Academy of Sciences of the USSR V. M. Vdovenko for providing the opportunity to carry out the magnetic measurements in his laboratory. The authors are also grateful to V. A. Shcherbakov for assistance in conducting this study.

Radium Institute named after V. G. Khlopin
Academy of Sciences of the USSR

Received
21 VII 1962

CITED LITERATURE

1. N. I. Blinova, V. M. Solntsev, Yu. M. Tolmachev, DAN, 140, No. 6, 1314 (1961).
2. P. Selwood, Magnetochemistry, IL, 1958, p. 13.
3. W. Tilk, W. Klemm, Zs. anorg. u. allgem. Chem., 240, 355 (1939).
4. E. Wedckind, C. Horst, Ber., 48, 111 (1955).
5. H. Haraldsen, K. Bakken, Naturwiss., 28, 127 (1940).
6. J. Katz, E. Rabinowitch, The Chemistry of Uranium, IL, 1954, p. 227.